Alternative way to do FirstOrDefault in LINQ

问题

I've got a membership table that records whether a user is a member of a list. When an update to a user's membership occurs a new record is written and the previous records are left as is which allows a history of their memberships to be maintained. To get a user's membership status involves selecting their most recent entry.

An example of some user list membership data is below. The aim is to find a LINQ expression that groups by list and user but only returns the row with most recently inserted record.

List Name, Username, Comment, ExpiresOn, Inserted
Test List, joeb, second update, 2012-03-13 16:55:03, 2012-01-31 22:28:40
Test List, joeb, first update, 2012-02-13 16:55:01, 2012-01-31 22:28:39
Test List, joeb, initial, 2012-01-13 16:55:02, 2012-01-31 22:28:38

An SQL query illustrates how the current list membership status can be extracted.

select ulm2.ID, ulm2.ExpiresOn, ulm2.Comment, ulm2.Inserted 
from UserListMembership as ulm1
left outer join UserListMembership ulm2 on ulm1.id = ulm2.id
group by ulm1.userlistid, ulm1.userid;

The question is how to write a LINQ expression for the query that doesn't use a nested FirstOrDefault call which causes my MySQL entity framework provider to throw a "System.NotSupportedException: Specified method is not supported." exception?

Update:

Below is my failing LINQ expression, it throws the "Specified method is not supported" once the FirstOrDefault call is added.

var query = from mem in db.UserListMemberships
            where
              mem.User.UserUsernames.Any(y => y.Username.ToLower() == username.ToLower())
              && mem.UserList.Account.Subscriptions.Any(x => x.ID == subscriptionID)
              && mem.ExpiresOn > utcNow
            group mem by new { mem.UserListID, mem.UserID } into g
            select new { UserListMembership = (from mem2 in db.UserListMemberships where mem2.UserListID == g.Key.UserListID && mem2.UserID == g.Key.UserID orderby mem2.Inserted descending select mem2).FirstOrDefault() };

 return query.Select(a => a.UserListMembership).ToList();

回答1:

Without posting code it's hard for anyone to see what your issue is.

Are you not able to just order the results by your date field and then select the first record? Something like users.OrderByDescending(u => u.Inserted).FirstOrDefault()?

来源：https://stackoverflow.com/questions/9088108/alternative-way-to-do-firstordefault-in-linq